The present invention relates to the locally resolved investigation of objects by means of magnetic resonance (MR) and particularly concerns a method of and an apparatus for the acquisition of data for an image representation which shows the spatial distribution of the MR behavior of an object within a selected localised region, according to the preamble of claim 1 and of claim 18 respectively.
In the conventional MR imaging methods, the object region to be investigated, i.e. the xe2x80x9cprobexe2x80x9d, is arranged in a stationary magnetic field B0 and a succession of at least one electromagnetic high frequency (HF) pulse of selected frequency and following pulses of magnetic field gradients are applied in different spatial directions, such that, as a consequence of the high frequency excitation, echoes appear which are detected as NMR signals and which give information as to the condition of the probe. In this connection, besides the density of the spin which can be influenced by the high frequency pulses, there are various characteristic relaxation time constants of the spin magnetisation, among others the spin-grid relaxation time T1, the spin-spin relaxation time T2 and the effective spin-spin relaxation time T2*. Mention should also be made of the time constant designated as T1, which describes the relaxation of the magnetisation in the direction of an effective magnetic field which is composed of static and a high frequency magnetic field. In other words, T1 describes the relaxation in a rotating coordinate system.
The energy content of the high frequency pulses determines the amount of the excited spin capable of emitting an MR signal (transversal magnetisation) in proportion to the spin present in the equilibrium condition (longitudinal magnetisation). The inverse tangent of this ratio is designated as the flip angle of the high frequency pulse.
The resonance frequency of the spin and consequently the frequency both of an excitable high frequency pulse and also of the measurable MR signals is determined by the localised magnetic field strength. For the localised resolution, therefore for all imaging methods, during the signal detection, a so-called read gradient is imposed in a chosen spatial direction, in order to associate different local regions along this direction with different frequencies in the signal (frequency coding). By a Fourier transformation, the different frequencies and consequently the contributions of different local regions can be separated. In this way, a localised resolution is possible in the relevant spatial direction, which is designated also as the xe2x80x9cfrequency axisxe2x80x9d.
In order to achieve localised resolution in a second spatial direction which is orthogonal to the read direction, it is conventional, before the appearance of the signal to be detected, to impose transiently a gradient in this direction, which has the effect of dephasing the oscillations (spins) excited in the probe along the relevant spatial direction. By stepped changing of the time integral of this xe2x80x9cphase gradientxe2x80x9d from echo to echo, the phase of the signal contribution originating from one local place changes from echo to echo. The signal contributions of the different places along this direction can be separated from one another by a Fourier transformation with reference to the current number of the echo. Since frequency and phase are separately dependent on the position along orthogonal spatial coordinates, a two-dimensional image of the object can be reconstructed.
A local selection in a third spatial direction is effected by applying a gradient in this direction during the exciting frequency-selective high frequency pulses. By this xe2x80x9cslice gradientxe2x80x9d a slice is selected in the object for the excitation.
The most common MR imaging methods work with the combined frequency and phase coding described above. For the representation for example of a two-dimensional N-line image, N echoes are produced one after another, each with a different phase coding and with each echo of this N echo sequence being. frequency coded in the same way by the read gradient and scanned as an MR signal. From the scanned values of the detected signals, a two-dimensional matrix of data is formed, the so-called K-space, each row or xe2x80x9clinexe2x80x9d of which has a different frequency coded echo associated therewith and contains scanned values of the relevant echo. The line direction is also designated as the frequency axis of the K-space. The axis of the K-space which is orthogonal to this is scaled as phase coordinates, i.e. the position of a row along this axis is defined by the integral of the phase gradients. The data matrix which is thus organised is then subjected to a two-dimensional Fourier transformation (2D-FT) in order to obtain the pixel values of the image.
Also, other less usual MR imaging methods (projection reconstruction imaging, spiral imaging) can be used to scan the 2D-K-space, where the strict separation between phase coding direction and read gradient direction is abolished in these methods. In general, with these methods, the K-space is scanned not equidistantly in non-rectangular trajectories. Therefore, for these methods, other image reconstruction methods must be used.
In the MR signals one must differentiate between three different types. The so-called xe2x80x9cspin echo signalxe2x80x9d arises from refocusing of the magnetic field inhomogeneity effects by means of an additional high frequency pulse which is applied for a certain time after the first high frequency excitation pulse. The so-called xe2x80x9cgradient echo signalxe2x80x9d is produced by polarity reversal of a magnetic field gradient (usually the read gradient), as a result of which there is a refocusing of the de-phasing brought about by the previous effect of this gradient. So-called xe2x80x9cstimulated echo signalsxe2x80x9d and echo signals of higher order arise after a succession of at least three high frequency pulses with flip angles which are not equal to 180xc2x0.
The total echo sequence (xe2x80x9cN echo sequencexe2x80x9d) required for the receipt of an N-line image can be produced by the most varied of MR sequences. Each MR sequence is composed of a single sequence or by multiple repetition of the same sequence of high frequency pulses and magnetic field gradient shifts.
The required N echo sequence can be produced by sequences consisting of an N-fold repetition of the same sequence, wherein each sequence consists of a single high frequency excitation pulse and a single echo, so-called 1-echo sequence, developed from a read gradient reversal (gradient echo) or a refocusing high frequency pulse in combination with suitable read gradient shifts (spin echo). Alternatively however, after a high frequency excitation pulse, several spin echoes and/or gradient echoes can be produced within a sequence, and can be coded for the image representation in the manner described above. One would speak here of multi-echo sequences (M-echo sequence). Depending upon whether one produces all required N echoes by means of one excitation and a single sequence, or whether the N echoes are collected in several successive sequences each with its own excitation pulse sequence, one speaks of a single-shot sequence or of multi-shot sequence methods.
In many applications of the MR imaging one is seeking to carry out the echo production and echo detection as rapidly as possible. In the last two decades, a large number of rapid imaging techniques have been proposed which are described extensively in the literature. Some of the methods described there have achieved wide use. From the methods conventional at the present time, the single-shot sequence variation of the so-called xe2x80x9cecho planar imagingxe2x80x9d (EPI) is the most rapid; here the whole total image information is obtained in a single sequence in the form of gradient echoes after a single excitation pulse by an ultra-fast sequence of read gradient reversals within 25 to 250 ms, so that image artefacts caused by movement are almost completely excluded. However, this method has the disadvantage of poor spatial resolution, since the number of the echoes measurable after the excitation pulse is limited because of the inherent rapid T2* relaxation. Moreover, this method imposes high demands in terms of hardware on the MR imaging system, particularly in respect of the homogeneity of the static magnetic field, the gradient strength, the gradient switching speed and the gradient amplification power.
For these reasons, in the past, special modified arrangements of the EPI method and other rapid but less critical methods have been proposed, which are described extensively in the literature and several of which have in the meantime proved to be preferable in practice. There follows a representative selection of base materials from the literature;
[1] P. Mansfield, xe2x80x9cMulti-planar formation using NMR spin echosxe2x80x9d, J. Phys. C. Solid State 10, L55-L58 (1977);
[2] J. Frahm, A. Haase, D. Matthaei, K.-D. Merboldt, W. Hxc3xa4nicke, xe2x80x9cRapid NMR imaging using stimulated echosxe2x80x9d, J. Magn. Reson. 65, 130-135 (1985);
[3] J. Hennig, A. Nauerth, H. Friedburg, xe2x80x9cRARE imaging: a fast imaging method for clinical MRxe2x80x9d, Magn. Reson. Med. 3, 823-833 (1986);
[4] A. Haase, J. Frahm, D. Matthaei, W. Hxc3xa4nicke, K.-D. Merboldt, xe2x80x9cFLASH imaging. Rapid NMR imaging using low flip-angle pulsesxe2x80x9d, J. Magn. Reson. 67, 258-266 (1986);
[5] A. Haase, xe2x80x9cSnapshot FLASH MRI. Applications to T1, T2, and chemical shift imagingxe2x80x9d, Magn. Res. Med. 13, 77-89 (1990);
[6] K. Oshio, D. A. Feinberg, xe2x80x9cGRASE (Gradient-and-Spin-Echo) Imaging: A novel Fast MRI Techniquexe2x80x9d, Magn. Res. Med. 20, 344-349 (1991);
[7] K. Oshio, D. A. Feinberg, xe2x80x9cSingle-shot GRASE imaging without fast gradientsxe2x80x9d, Magn. Res. Med. 26, 355-360 (1992);
[8] D. A. Feinberg, B. Kiefer, G. Johnson, xe2x80x9cGRASE Improves Spatial Resolution in Single Shot Imagingxe2x80x9d, Magn. Res. Med. 33, 529-533 (1995);
[9] J. Hennig, M. Hodapp, xe2x80x9cBurst imagingxe2x80x9d, MAGMA 1, 39-48, (1995);
[10] I. J. Lowe, R. E. Wysong, xe2x80x9cDANTE ultrafast imaging sequence (DUFIS)xe2x80x9d, J. Magn. Res., Series B 101, 106-109 (1993);
[11] P. Margosian, F. Schmitt, D. E. Purdy, xe2x80x9cFaster MR imaging: Imaging with half the dataxe2x80x9d, Health Care Instr. 1, 195-197 (1986);
[12] D. Feinberg, J. Hale, J. Watts, L. Kaufmann, A. Mark, xe2x80x9cHalving MR Imaging Time by Conjugation: Demonstration at 3.5 kGxe2x80x9d, Radiology 162, 527-531 (1986);
[13] G. C. McKinnon, xe2x80x9cUltrafast interleaved gradient-echo-planar imaging on a standard scannerxe2x80x9d, Magn. Res. Med. 30, 609-616 (1993);
[14] S. Dang, J. B. Weaver, J. F. Dunn, xe2x80x9cA hybrid fast imaging method of RARE and BURST/QUESTxe2x80x9d, in Proc. SMR 2nd Annual Meeting, San Francisco, 1994, page 487;
[15] P. van Gelderen, C. T. W. Moonen, J. H. Duyn, xe2x80x9cSusceptibility Insensitive Single Shot MRI Combining BURST and Multiple Spin Echosxe2x80x9d, Magn. Res. Med. 33, 439-442 (1995);
[16] D. K. Sodickson, W. J. Manning, xe2x80x9cSimultaneous Acquisition of Spatial Harmonics (SMASH): Fast Imaging with Radiofrequency Coil Arraysxe2x80x9d, Magn. Res. Med. 38, 591-603 (1997);
[17] K. P. Prxc3xcssmann, M. Weiger, M. B. Scheidegger, P. Boesiger, xe2x80x9cCoil Sensitivity Encoding for Fast MRIxe2x80x9d, ISMRM 6th Annual Meeting, page 579 (1998);
[18] M. Hutchinson, U. Raff, xe2x80x9cFast MRI data acquisition using multiple detectorsxe2x80x9d, Magn. Res. Med. 6, 87-91 (1988);
[19] J. W. Carlson, T. Minemura, xe2x80x9cImaging time reduction through multiple receiver coil data acquisition and image reconstructionxe2x80x9d, Magn. Res. Med. 29, 681-688 (1993);
[20] J. B. Ra, C. Y. Rim, xe2x80x9cFast imaging using subencoding data sets from multiple detectorsxe2x80x9d, Magn. Res. Med. 30, 142-145 (1993);
[21] A. E. Holsinger, S. J. Riederer, xe2x80x9cThe importance of phase encoding order in ultra-short TR snapshot MR imagingxe2x80x9d, Magn. Res. Med. 16, 481-488 (1990);
[22] R. V. Mulkern, S. T. S. Wong, C. Winalski, F. A. Jolesz, xe2x80x9cContrast manipulation and artifact assessment of 2D and 3D RARE sequencesxe2x80x9d, Magn. Reson. Imag. 8, 557-566 (1990);
[23] D. R. Bailes, D. J. Gilderdale, G. M. Bydder, A. G. Collins, D. N. Fermin, xe2x80x9cRespiratory Ordering of Phase Encoding (ROPE): a method for reducing respiratory motion artifacts in MR imagingxe2x80x9d, J. Comput. Assist. Tomogr. 9(4), 835-838 (1985);
[24] E. M. Haacke, J. L. Patrick, xe2x80x9cReducing motion artifacts in two-dimensional Fourier transform imagingxe2x80x9d, Magn. Reson. Imaging 4, 359-376 (1986);
[25] H. W. Korin, S. J. Riederer, A. E. H. Bampton, R. L. Ehmann, xe2x80x9cAltered Phase-Encoding Order for Reduced Sensitivity to Motion in Three-dimensional MR Imagingxe2x80x9d, JMRI 2, 687-693 (1992);
[26] C. K. Macgowan, M. L. Wood, xe2x80x9cPhase-Encode Reordering to Minimize Errors Caused by Motionxe2x80x9d, Magn. Res. Med. 35, 391-398 (1996);
[27] M. Weiger, P. Bxc3x6rnert, R. Proska, T. Schxc3xa4ffter, A. Haase, xe2x80x9cMotion-Adapted Gating Based on k-space Weighting for Reduction of Respiratory Motion Artifactsxe2x80x9d, Magn. Res. Med. 38, 322-323 (1997);
[28] M. Fuderer, xe2x80x9cThe information content of MR imagesxe2x80x9d, IEEE Trans. Med. Imaging 7, 368-380 (1988);
[29] J. J. van Vaals, M. E. Brummer, W. T. Dixon, H. H. Tuithof, H. Engels, R. C. Nelson, B. M. Gerety, J. L. Chezmar, J. A. den Boer. xe2x80x9cxe2x80x98Keyholexe2x80x99 Method for accelerating Imaging of Contrast Agent uptakexe2x80x9d, JMRI 3, 671-675 (1993);
[30] D. A. Feinberg, K. Oshio, xe2x80x9cPhase Errors in Multi-Shot EPIxe2x80x9d, Magn. Res. Med. 32, 535-539 (1994);
[31] F. Hennel, xe2x80x9cMultiple-Shot Echo-Planar Imagingxe2x80x9d, Concepts in Magn. Reson. 9(1), 43-58 (1997);
[32] B. Chapmann, R. Turner, R. J. Ordidge, M. Doyle, M. Cawley, R. Coxon, P. Glover, xe2x80x9cReal-Time Movie Imaging from a Single Cardiac Cycle by NMRxe2x80x9d, Magn. Res. Med. 5, 246-254 (1987);
[33] R. R. Edelman, W. J. Manning, D. Burstein, S. Paulin, xe2x80x9cBreath-Hold MR angiography of human coronary arteriesxe2x80x9d, Radiology 181, 641-643 (1991);
[34] D. J. Atkinson, R. R. Edelman, xe2x80x9cCineangiography of the heart in a single breath-hold with a segmented turboFLASH sequencexe2x80x9d, Radiology 178, 357-360 (1991);
[35] P. M. Jakob, M. Griswold, K. O. Lxc3x6vblad, Q. Chen, R. R. Edelmann, xe2x80x9cHalf-Fourier BURST Imaging on a clinical scannerxe2x80x9d, Magn. Res. Med. 38 (4), 534-540 (1997);
[36] J. P. Mugler III, xe2x80x9cPotential Degradation in Image Quality Due to Selective Averaging of Phase-Encoding Lines in Fourier Transform MRIxe2x80x9d, Magn. Res. Med. 19, 170-174 (1991);
[37] C. T. W. Moonen, G. Lia, P. van Gelderen, G. Sobering, xe2x80x9cA Fast Gradient-Recalled MRI-Technique with Increased Sensitivity to Dynamic Susceptibility Effectsxe2x80x9d, Magn. Reson. Med. 26, 184-189 (1992).
Reference is made hereinafter to a number of these literature references by the use of the identifying number in square brackets [ ].
The rapid MR imaging techniques which are presently being discussed or used in the technical world can be divided roughly into five categories:
(a) Conventional multi-shot sequence methods (e. g. standard spin echo techniques) which fill few points in the K-space [11, 12]. The quite modest shortening of the total measuring time (by a factor of 2 to 4 compared with normal multi-shot sequence spin echo methods) is paid for by a corresponding reduction in the spatial resolution.
(b) Multi-shot sequence methods with flip angles  less than 90xc2x0, gradient echo and short repetition time  less than  less than T1 (FLASH methods and variations thereof [4, 5]). With this one can achieve a reduction of the total measuring time by a factor of 10 to 1000 as compared with normal multi-shot sequence spin echo methods. These multi-shot sequence methods require, in comparison to the EPI imaging, an increased number of gradient switching points and as a consequence of this an increased total image measuring time with simultaneously reduced signal-to-noise ratio (S/R). This method offers advantages in respect of the distortion-free representation of object regions where there is poor magnetic field homogeneity and the robust representation of movement and flow.
(c) Single-shot sequence methods; for this one is talking about the aforementioned EPI [1], spin echo methods with a rapid succession of echoes through direct succession of the refocusing high frequency pulses (RARE [3]), or methods in which special high frequency excitation pulses are used in the presence of a constant magnetic field gradient, which produce a plurality of echo signals (BURST [9, 10]). With these methods, total measuring times of the order of 10 to 500 ms are achieved. In these single-shot sequence methods, basically the maximum localised resolution and also the achievable signal-to-noise ratio is limited by signal losses, caused by relaxation time effects and diffusion effects. Additionally, image artifacts can occur due to flow and/or movement.
(d) Hybrid methods [6-8, 13-15], in which either several equal multi-echo sequences are repeated (e. g. multi-shot EPI [13]) or in which each spin echo of a single-shot sequence (RARE) is xe2x80x9csplitxe2x80x9d by read gradient reversals into a plurality of gradient echoes (GRASE [6-8]). These methods permit total measuring times of the order of 100 ms to 30 s and offer the advantage of low signal losses and consequently a higher signal-to-noise ratio. Disadvantages include the signal modulation due to this detection method, which can result in ghost images, and also the increased sensitivity to measurement errors due to flow and movement.
(e) Parallel methods in which different signal reception coils are used simultaneously in order to fill different lines of the K-space in simultaneous manner. Such methods (e. g. SMASH [16] or SENSE [17]) can be performed with almost all existing imaging sequences and bring about at the same time an additional reduction of the total measuring time by a factor of about 2 to 8. The disadvantage of these parallel methods lies in the fact that they are up to now not yet technically mature.
An important common feature of all the MR sequence categories (a)-(d) known today is that all MR sequences can be combined as a succession of one or more approximately identical sequences, wherein for the case of the multi-shot sequence methods, each sequence differs only through the degree of the phase coding and other trifling changes from the preceding or subsequent sequence (e.g. changes in the time structure, and also echo time shifting are known, cf. [30, 31]). Thus, for example the FLASH method can be described as a multi-shot sequence method, since this is combined from identical sequences of high frequency pulses, slice gradient and read gradient, in which a phase gradient is increased incrementally in steps only from sequence to sequence. An EPI sequence can correspondingly be defined as a single-shot sequence method, in which the whole of the image information is received in a single sequence. Segmented EPI methods or GRASE methods can likewise be defined as multi-shot sequence methods according to the classification here used, since these likewise combine identical sequence blocks of high frequency pulses, slice gradient and read gradient, in which the value of the phase gradient which is used is changed only from sequence to sequence.
Each of the four method categories (a) to (d) has its own advantages and disadvantages. Each method which offers a particular advantage also shows a series of disadvantages. Different aspects of the image quality, such as contrast, sharpness, signal-to-noise ratio (S/N), contrast-to-noise ratio (C/R), spatial resolution and the occurrence of certain artifacts are emphasized or attenuated to different degrees with the different sequences. Thus, with use of a sequence which emphasises one or more particular aspects of the image quality, one can count on a degradation of at least one of the other aspects. What is practically incontrovertible with the state of the art up to now is the fact that measures which are used for shortening the total measuring time and consequently for accelerating the imaging are often accompanied by a marked degradation of the image quality. This applies to almost all aspects of the image quality. However, artifacts which arise due to movement of the object are often reduced by acceleration of the acquisition of the data.
It is the object of the present invention to provide a method for the acquisition of data for the MR imaging such that a better compromise is achieved than heretofore between various aspects of the image quality or between the speed of data acquisition and the desired aspects of the image quality. This object is achieved in accordance with the invention by the features set out in claim 1. Particular embodiments and developments of the invention are set out in the subsidiary claims. The features of an apparatus according to the invention are set out in claim 18.
With the invention, one can use in a novel manner the realisation that the contrast of an MR image is chiefly determined by the information of low spatial frequency which resides in the middle of the K-space, because the image energy is more strongly concentrated in the center of the K-space. This applies equally to the signal-to-noise ratio and the contrast-to-noise ratio. In the outer regions of the K-space there is a greater frequency of information which contributes more to the resolution of the MR image at its borders. A further effect which is utilisable in a novel manner with the invention is that movement artifacts are so much smaller the further that MR signals received during significant movements are remote from the center of the K-space, cf. [23] to [27].
Correspondingly, the principle of the invention lies in the fact that different conditions are produced in the course of the signal production, because the sequence of at least two different sequences of high frequency pulses and gradient pulses succeeding each other in time are combined, with each sequence differing in at least one of the features of the echo production which is responsible for different aspects of the image quality. The echo signals produced in this way by different sequences are then combined in their own bands of the K-space. Preferably, in order to emphasise a desired aspect of the image quality, a sequence is chosen which is optimum for that aspect, but only the middle band of the K-space is filled with the scanned MR data (echo scan values) of this sequence. The remaining part of the K-space, which contributes less intensively to the overall appearance of the image, is filled on the other hand with the echoes of another sequence which may lay less stress on the particular chosen aspect, but emphasize some other aspect which has a particularly favorable effect on the borders of the K-space. The aforesaid other sequence can be a more rapid sequence in order thus to achieve a measuring time which is shorter overall, without the desired image quality being noticeably poorer and without moving object regions leading to strong artifacts.
The inventive principle is a hybrid method which differs from the known hybrid methods already mentioned above in that different types of sequence and possibly different speed sequences are performed one after another in time and the signals of the different sequences are allocated to separate bands of the K-space. A well-directed, band-type classification of echo groups, which appear in preselected groups of time windows within the total sequence, in pre-selected bands of the K-space, is known it is true in connection with the GRASE method (so called k-banded GRASE), but only within the framework of a single sequence or an unchanged repetition of that sequence. In relation to GRASE it should be mentioned for the sake of completeness that there echo signals of different types are produced which are interleaved in time within each individual sequence, and then can be classified in terms of bands or in some other way in the K-space, and in which the sequence is repeated many times without change.
In spite of the large multiplicity of common MR imaging sequences and in spite of the many endeavors reflected in the aforementioned literature, to produce hybrid forms, a combination method comparable with that of the present invention has until now not been proposed. The reason for this is thought to be the fact that a time-wise switching over between different MR sequences with different signal and contrast characteristics for the formation of a total sequence has not been considered to be feasible technically and moreover would lead to significant image artifacts. It has now been found however that this is a prejudice which has been overcome with the present invention.
A possible further optimising step in a method according to the invention includes the use of the principle of mixed bandwidth, wherein a sequence-dependent change of the bandwidth of the echo readout is brought about. For this, the association of the different bandwidths with the echoes in the K-space read out with different sequences is likewise dependent upon which aspects of the image quality should be emphasized. If for example it is the wish that there should be an edge emphasis in the foreground, then higher readout bandwidths are to be used for the middle region of the K-space than for the outer regions.
The principle of an image sequence according to the invention permits the writing of different sequences of high frequency pulses and magnetic field gradient pulses united in one succession (and thus connected with a change in the echo production and/or changes in the received type of echo), which if planned can be combined also with a change of the bandwidth in the echo readout.
For the practical realisation of a method according to the invention, the skilled person can proceed in the following manner:
1. As the first step, a sequence is chosen as the xe2x80x9cmain sequencexe2x80x9d, which has the advantage of being able to produce the desired image contrast in the special particular application.
2. As the second step, at least one other sequence is chosen as the xe2x80x9cauxiliary sequencexe2x80x9d, which has another advantage which is less well achievable than with the first mentioned sequence.
3. In an optimisation step, a proportioning factor xcex is chosen, which determines how wide the middle band of the K-space which is to be filled by the echo signals of the main sequence should be in comparison with the total width of the K-space. By the optimisation, it can also be decided whether and in what way the bandwidth of the echo readout is changed upon filling of the K-space.
4. The main and auxiliary sequences are carried out one after the other in time (in any order), wherein the echo signals of the main sequence are written into the middle region (xcex) and the echo signals of the auxiliary sequence are written into the remaining part (1-xcex) of the K-space, possibly with varying bandwidth.
The possibility of carrying out an optimisation by variation of the relative widths of the bands of the K-space which are respectively to be filled is an important advantage of the principle of the present invention. The possibility of optimisation by varying the readout bandwidth relieves the sequence designer from the obligation to use equal width time windows (and consequently also equal strength read gradients) for the echo readout.